1. Field of the Invention
The present invention relates to an image forming apparatus forming an image on a recording material, for example an image forming apparatus such as a copier, printer or fax machine.
2. Related Background Art
Most fixing apparatuses which have been employed in electrophotographic apparatuses are hot-press type fixing apparatuses in which a toner carried on a recording material is heated and pressured to be melted and fixed.
They are classified broadly into two types: one is a roller type fixing apparatus (roller fixing) in which a pair of rollers are abutted against each other in a face-to-face manner, heating sources are placed in one of the rollers or both rollers, and an abutment portion thereof is made to nip and convey a recording material to execute fixing process, and the other is so called a belt type fixing apparatus (belt fixing) comprised of a roller as one part and a belt as the other part.
That is, it has as a basic structure a roller pair of a fixing roller (heat roller) thermally adjusted to have a predetermined fixing temperature and a pressure roller abutted against the fixing roller, wherein the roller pair is rotated, a recording material having an unfixed toner image formed thereon is introduced into a fixing nip portion being an abutment portion of the roller pair, and nipped and conveyed to hot pressing-fix the unfixed toner image on the recording material by means of the heat of the fixing roller and the pressure of the fixing nip portion.
The amount of heat given to the recording material bearing the unfixed toner image depends on temperatures of the fixing roller and the pressure roller, and a time period over which the recording material passes through the fixing nip portion, i.e. a fixing nip width and a travel speed of the recording material. The fixing nip width refers to a length of the fixing nip portion along which the recording material travels.
For a fixing apparatus to be mounted on an electrophotographic apparatus having a higher process speed, a fixing apparatus having a larger fixing nip width is required in relation to the amount of heat described above. For increasing the fixing nip width in the roller fixing described above, the diameter of the roller should be increased, and if the diameter of the roller is increased, there arises a problem such that the heat capacity of the roller increases to lengthen the heat-up time (warm-up time) of the roller.
Thus, as a fixing apparatus configuration capable of securing a large fixing nip width without increasing the diameter of the roller, a belt type fixing apparatus is proposed in Japanese Patent Application Laid-Open No. S61-132972.
In such a belt type fixing apparatus, a heat-resistant and flexible endless belt stretched between a plurality of belt stretching members is abutted against a fixing roller being a rotating member for heating to form a fixing nip portion, and the fixing nip portion is made to nip and convey a recording material bearing an unfixed toner image, whereby the unfixed toner image is hot press-fixed on the recording material by means of the heat of the fixing roller and the pressure of the fixing nip portion. This belt type fixing apparatus allows the fixing nip width to be easily set to a larger width by adjusting a width of the endless belt facing the fixing roller. The fixing nip width can be secured independently of the diameter of the fixing roller, and therefore the diameter and the heat capacity of the fixing roller can be reduced, thus making it possible to shorten the start-up time.
In the case of the belt type fixing apparatus, so called a fixing nip width is large from a configuration viewpoint, a time period over which the fixing roller and the belt contact each other is therefore lengthened, and thus the temperature of the belt tends to be elevated. The heat capacity of the belt is small compared to the pressure roller, and therefore the temperature tends to be elevated in a short time period. Thus, the following problems arise.
(1) The amount of heat given to the recording material increases, and therefore the amount of water vapor emerging from the recording material containing water increases. The water vapor is blocked by a toner layer fixed on the front surface of the recording material, thus can hardly escape to the front surface of the recording material, and concentrates on the rear surface of the recording material. Thus, a water vapor layer with a pressure high enough to separate the recording material and the pressure belt tends to be formed between the rear surface of the recording material and the belt in the fixing nip portion, and this layer drastically reduces a frictional force at the interface between the rear surface of the recording material and the belt. As a result, there are cases where a recording material conveying power with rotation of the belt is considerably reduced, leading to occurrence of an image defect and a conveyance defect.
(2) Some users use, as the recording material, resin coated sheets (hereinafter referred to as coated sheets) of which the glossiness of the front surface or both front and rear surfaces is increased by coating the front surface or both front and rear surfaces with an acryl resin, polyolefin resin or the like. There are cases where when the fixing apparatus is made to pass with a toner placed on the coated sheet, an image defect called a blister in which the recording material is partially deformed like a blister occurs. It can be considered that this is caused due to the fact that water in the interior of the coated sheet is evaporated by heating by the fixing apparatus to increase the volume, but due to a coat layer on the surface of the coated sheet, vaporized water vapor is not uniformly dissipated to outside the coated sheet, but collectively discharged to outside the coated sheet from a portion in which the coat layer is thinned or absent, and therefore the coat layer is ruptured.
For preventing occurrence of such a phenomenon, a configuration in which the belt and the fixing roller are separated during standby is proposed in Japanese Patent Application Laid-Open No. H11-194647.
However, the above conventional technique has the following problem.
In an image formation job for continuously forming images on a plurality of recording materials, the belt contacts the fixing roller even during a time period over which no recording material exists in the fixing nip (so called sheet interval), and heat of the fixing roller is therefore transferred to the belt, and the temperature of the belt becomes equivalent to the temperature of the fixing roller. Thus, fixing process can be satisfactorily executed for recording materials in the initial stage of the image formation job, but there are cases where the above image defect and conveyance defect resulting from elevation of the temperature of the belt occur for recording materials in the middle or late stage of the image formation job.